1. Technical Field
This invention relates generally to laser systems and, more particularly, to a polarizing free-electron laser system and polarization conversion for producing azimuthal and radial polarization laser beams.
2. Discussion
The polarization of a laser beam is a characteristic in which the electric field thereof is controlled with respect to the propagation direction of the beam. Different types of polarization exist which are selected so that the electric field is oriented in a predictable fashion, depending on a particular application. Conventional laser systems have been developed which commonly generate a linearly polarized beam. According to these conventional approaches, the linear polarization is generally accomplished by employing a planar Brewster window oriented at the Brewster angle (i.e., polarizing angle) within the laser region. The planar Brewster window is usually made of a transparent dielectric material such as glass or dichroic material which has a known index of refraction. Accordingly, light with a polarization parallel to the incidence plane of the planar Brewster window is transmitted therethrough, while light with a polarization normal thereto is partially reflected there from.
In the past, other types of polarization have been produced which include circular and elliptical polarization. More recently, developments in the areas of holography, interferometry, spectroscopy, photochemistry and accelerator technology now and in the future may require laser beams in a special polarization state known as radial polarization. In particular, a radially polarized laser beam can advantageously be focused by axicon-type optics so as to generate a very strong longitudinal electromagnetic field in the focal region thereof. In effect, radially polarized laser beams could therefore be used to provide an accelerating mechanism for present and future-generation accelerators such as high energy (i.e., gigavolt) accelerators.
Currently, existing laser systems have been able to generate radial polarization beams using very complex optical schemes. These schemes generally involve converting a linearly polarized beam into a radially polarized beam through a series of beam rotations and combinations with external conversion optics. However, existing external conversion systems are rather complex since the conversion generally requires special optical elements such as a spiral waveplate which is very difficult to fabricate in the optical region. In addition, prior approaches generally further require a substantially uniform beam profile which results in rather stringent requirements.
One particular type of laser known as the free-electron laser (FEL) generally involves the passage of a beam of electrons through a spatially varying magnetic field which in effect causes the electron beam to wiggle and hence to radiate. Free-electron lasers generally include a wiggler configuration which affects the electron motion and therefore the polarization of the laser beam. According to conventional wiggler approaches, the linear wiggling motion of electrons is able to generate a linear polarization optical field, while a circularly orbiting electron beam radiates a circularly polarized optical field.
While conventional free-electron lasers have shown the ability to produce linear and circular polarization, the generation of other types of polarization generally require complex electron beam focusing arrangements. For instance, in order to generate radial polarization, the electron beam in a conventional free-electron laser generally has to be focused and defocused in a distance of a wiggler period so that the electrons have the same radial motion simultaneously. However, it is generally difficult to focus the beam to have a significant radial velocity component due to limiting factors such as the space charge effect. As a result, tile direct generation of a radial polarization beam from a free-electron laser is usually not very practical.
It is therefore desirable to provide for an enhanced approach for obtaining a laser output beam having a radial polarization. In addition, it is desirable to provide for a laser system which generates a radial polarization output beam without the need for any overly complex external conversion optics. Furthermore, it is desirable to provide an optical polarization conversion for converting azimuthally polarized optical radiation to radially polarized optical radiation so that a free-electron laser may indirectly produce radial polarization therewith. It is further desirable to provide for such an optical polarization conversion for converting between azimuthal and radial polarization optical radiation.